After a recent City Hall report showed a steep drop in recycling for the last year, city officials blamed the struggling newspaper industry. Now, they have changed their tune.

A drop in circulation, they said in the mayor’s management report released earlier this month, resulted in residents leaving fewer newspapers at the curb, leading to less paper being recycled.

The decrease in recycling is raising concerns among environmental advocates and the City Council, who said they doubted that falling circulation could cause a 20% citywide decline. Newspaper sales have been dropping for years, but recycling had been increasing in the city until the most recent year.

“That’s an old saw that they’ve been playing for four years. I just don’t buy into that,” the chairman of the council’s Sanitation Committee, Michael McMahon, said. Mr. McMahon said he would convene a hearing on the recycling decline later this fall.

The Sanitation Department has backed off the rationale it offered in the mayor’s management report, and a closer look at city data confirms that sagging newspaper sales had little to do with the drop-off in recycling.

While the total tonnage of recycled material fell by nearly 20%, the average daily amount of paper picked up curbside dropped only 4.6%, to 1,274 tons a day. The amount of metal, glass, and plastic recycled experienced an even smaller decline, 1.5%, to 780 tons a day.

The Sanitation Department attributes the majority of the recycling decrease to a sharper drop in the amount of bulk recycled materials used in landfill and construction projects, such as the conversion of the shuttered Fresh Kills landfill in Staten Island. A department spokeswoman, Kathy Dawkins, said that instead of using clean fill acquired from private contractors — which annually adds thousands of tons to the city’s recycling total — officials accepted material that the U.S. Army Corps of Engineers scooped up while dredging the Arthur Kill. The material was not included in the city’s recycling statistics.

Outweighing the newspapers, cans, and bottles collected curbside, the bulk items make up more than half of the total tonnage recycled each year.

The 20% decrease dropped the city’s total recycling to its lowest level of the Bloomberg administration, and means that the city fell well short of its targets for last year. The Sanitation Department had set a goal of recycling 22% of the city’s curbside waste and 37.6% of its total waste, but reached only 16.4% and 31.5%, respectively.

“It just shows me that they don’t care about recycling as much as they should,” Mr. McMahon said.

The city has slipped in its advertising of the recycling program, contributing to a lack of awareness and participation among residents, a senior lawyer for the Natural Resources Defense Council, Eric Goldstein, said. Recycling, he said, should be treated as a brand. “Coca-Cola is known throughout the world, and yet every year it spends a tremendous amount of money on advertising,” Mr. Goldstein said. “The same needs to be done for the recycling program.”

It’s been a rollercoaster ride for recycling since Mayor Bloomberg took office in 2002. He temporarily suspended the recycling of glass and metal to close a budget gap, and the city incrementally rebuilt the program before it was fully restored in 2004. But the frequent changes have caused confusion for New Yorkers, and Mr. Goldstein said it was a victory that recycling had kept up as much as it had.

Concerns over last year’s fall-off, however, are tempered by optimism regarding new recycling initiatives included in the 20-year trash plan approved by the council in July. The so-called Solid Waste Management Plan creates a separate Office of Recycling Outreach and Education within the mayor’s Council on the Environment. It also expands the types of plastic the city will recycling and creates a pilot program for recycling in public spaces such as parks, transit stations, and busy streets.

“There’s a widespread feeling those numbers will turn around in the next year or so,” Mr. Goldstein said, referring to the new programs.

The dip in recycling has led to a renewed call for food waste disposers, which advocates say would cut down on the amount of waste the city would have to export out-of-state. The push has been slowed by concerns over the environmental implications of the disposers. “I support a study,” Mr. McMahon said. “It makes sense, but I want to know the impact it will have on our sanitation system.”